Lagrangian measurement of fluid and particle motion using a field‐deployable Volumetric Particle Imager (VoPI)

Abstract

We describe the development of a field-deployable Volumetric Particle Imager (VoPI) and the methods that enable it to measure three-dimensional particle tracks in situ. The VoPI has the unique ability to obtain threecomponent particle velocity records in a volume. The device has a slender, single-camera design ideal for optically accessing study sites in the environment, including hard-to-reach places like the inside of coral reefs and marshes. Unlike other underwater measurement techniques, the VoPI implements a defocused imaging method that relies on ambient light in place of laser light for particle illumination. We describe the construction, calibration and validation of this device. We also demonstrate the VoPI's abilities by measuring velocity statistics of passive tracer particles dispersed in a high-Reynolds-number turbulent flow. By measuring velocity variances and integral timescales from these Lagrangian velocity records, we can compute the turbulent diffusivity directly from the VoPI measurements.

Wind‐driven water motions in wetlands with emergent vegetation

“…Such patterns are typically caused by coherent vortex structures that inject high‐momentum fluid downward (“sweeps”) and lift low‐momentum fluid upward (“ejections”) [ Pope , ]. The ‐ correlation was much weaker in the water, suggesting a lack of coherent vortex structures in the vertical‐although some of the decorrelation must be attributed to measurement noise inherent to the VoPI itself [ Tse and Variano , ]. This measurement noise (which is most evident at very low velocities) is random and is strongest in the vertical component due to anisotropy in the stereoscopic reconstruction.…”

“…Water velocity was measured using the Volumetric Particle Imager (VoPI), an optical tool designed in‐house to measure fluid velocities outdoor in hard‐to‐reach environments. Details of the VoPI's inner workings and performance can be found in Tse and Variano [], but in brief, it is a video recording system designed to record 3‐D particle tracks within a cubic‐centimeter sample volume. The VoPI was fastened to the bottom of the aluminum frame via a two‐axis stage mount that allowed translation along the vertical axis and one horizontal axis.…”

“…Images recorded by the VoPI were processed using the algorithms and procedures described in Tse and Variano [ Tse and Variano , ], giving three‐dimensional (3‐D) positions of suspended particles imaged in each frame. Particle trajectories could then be computed by linking corresponding particles across successive images.…”

Single-camera three-dimensional tracking of natural particulate and zooplankton

“…To measure volumes with a total depth of O(10 cm) without sacrificing depth resolution using the DDPIV technique, multiple cameras instead of multiple apertures on a single camera have been used to increase the effective distance between apertures d, thereby increasing the depth-resolution [23][24][25]. Single camera applications have been limited to volume depths of O(10 µm) − O (1 cm) [26][27][28][29][30][31][32][33][34]. However, as described above, the desired application to diver-operated ocean measurements necessitates a single-camera system capable of volumes with a depth of O (10 cm).…”

“…Neither of the approximations is appropriate for natural particulates and zooplankton in the ocean. Previous work has begun to address these concerns, such as a 1D cross-correlation method to determine the separation between image pairs of asymmetrically shaped particles [34]. Additionally, as objects span more pixels (e.g.…”